CARACAS, Venezuela (AP) — Venezuela’s oil minister said Monday that sabotage caused an explosion and fire last year that killed more than 40 people at the country’s main oil refinery, saying someone deliberately loosened bolts and released highly flammable gas.

The minister, Rafael Ramirez, did not say whether anyone specifically was suspected and ruled out employees of the state-owned PDVSA oil company. Separately, President Nicolas Maduro blamed the political opposition, although without providing evidence.

A former PDVSA security chief questioned Ramirez’s explanation, calling it speculative and saying it raised questions about why the leak wasn’t detected.

Shortly after the Aug. 25, 2012, conflagration at the Amuay refinery, reports emerged of faulty maintenance at the facility including dozens of accidents in the months before the disaster.

Ramirez alleged the blaze was caused by the loosening of seven bolts at a pump, releasing gas that exploded when National Guard troops stationed at the refinery started up vehicles nearby to evacuate.

He said the disaster caused $1.1 billion in damage. It took four days to extinguish the fire, and 42 people died and five were reported missing by official count. Only recently has the refinery restored production to 645,000 barrels per day of crude.

The former PDVSA security chief, Gustavo Benitez, said he found it difficult to believe that insurers would pay for damages caused by the disaster based on Ramirez’s explanation.

Benitez said that “the pump would have had to have been damaged, the sensors (that detect leaks) would have had to have been damaged” and mitigation systems as well. He said it appeared, rather, that “maintainence had been highly inefficient.”

Maduro’s claim that the opposition was involved in alleged sabotage follows his repeated blaming of political rivals for Venezuela’s ills. Since winning election in April by a razor-thin margin, the hand-picked successor of the late President Hugo Chavez has accused the opposition of sabotaging the overstrained power grid, causing food shortages through hoarding and mounting four alleged plots to assassinate him.

In no instance has Maduro substantiated the claims.

Last week, he claimed opposition sabotage was behind a failure in the country’s main electrical transmission line that caused about 70 percent of the nation to lose power for more than a half day.

Maduro on Monday predicted that “a war plan against the country will increase” in coming weeks. Elections are to be held Dec. 8 for mayors and municipal councils.

Political opponents led by Henrique Capriles, who insists Maduro stole the April 14 presidential election through fraud, scoff at his claims of sabotage. They say he is making them a scapegoat for his government’s inadequacies and his waning popularity — and to cover up corruption in this country with the world’s biggest proven oil reserves.

The Amuay disaster has raised questions about whether PDVSA has neglected maintenance while funneling revenues into the social programs that have made the socialist Chavistas popular with the poor.

A report done for an insurance carrier published widely right after the disaster and obtained by The Associated Press found failures in the complex’s maintenance and listed dozens of accidents.

It said the refinery had 222 accidents in 2011, including 100 fires mostly caused by breaks and leaks in pipes carrying combustible liquids.

Lawmaker Maria Corina Machado, who belongs to an opposition commission that is investigating the Amuay disaster, said via Twitter on Monday that PDVSA’s accident rate is 12 times the world average.

Critics say that in addition to refinery failures, PDVSA’s operations have suffered from the firing of nearly 18,000 oil workers in 2003, which was about 45 percent of the payroll, after they joined a strike called by Chavez’s political opponents to press demands that the president resign.